Fix the issue when provide a UIAnimatedImage and encode it into a static WebP failed

[dreampiggy]

Add a backup check of image.images.

See the demo code to trigger this issue:

UIImage *animatedImage = [UIImage sd_imageWithGIFData:gifData];
NSData *staticWebP = [SDImageWebPCoder.sharedCoder encodedDataWithImage:animatedImage format:SDImageFormatWebP options:@{SDImageCoderEncodeFirstFrameOnly : @(YES)}]
NSParamAsset(staticWebP);